Modified slurry compositions for forming improved chromium diffusion coatings

ABSTRACT

Unique and improved chromium coatings derived from modified chromium-containing slurry formulations are disclosed. The slurry formulation includes a combination of a selected halide activator and buffer material that synergistically interact with each other to form chromium diffusion coatings with improved microstructure in comparison to chromium diffusion coatings produced from conventional chromizing processes. The coatings may be locally applied in a controlled manner with accuracy onto various parts, including those having internal sections with complex geometries, without masking any portion thereof.

CROSS REFERENCE TO RELATED APPLICATION(S)

This application is a divisional application of allowed U.S. patentapplication Ser. No. 14/592,293 filed Jan. 8, 2015, and claims thebenefit of priority to U.S. provisional application Ser. No. 61/927,180filed on Jan. 14, 2014, and entitled MODIFIED SLURRY COMPOSITIONS FORFORMING IMPROVED CHROMIUM DIFFUSION COATINGS.

FIELD OF THE INVENTION

The present invention relates to novel and improved chromium diffusioncompositions and coatings that provide corrosion resistance ontometallic substrates.

BACKGROUND OF THE INVENTION

The components in the hot sections of gas turbine engines aresusceptible to degradation by hot corrosion attack. Hot corrosion canconsume the construction material of turbine engine components at anunpredictably rapid rate, and consequently lead to failure or prematureremoval of turbine engines. Hot corrosion typically occurs at atemperature range of about 650-950° C.

Molten deposits, such as alkali metal sulfates from intake air orcombustion of fuels, are the primary source of hot corrosion. However,other corrosive species such as sulfur dioxide in the environment canaccelerate the corrosion attack.

Hot corrosion that is sulfate induced, particularly Type II, has emergedas a concern for engine operation. Many of today's superalloys are moresusceptible to Type II corrosion, as they have lower levels of chromium,which as will be explained below, is known to be an effective alloyingelement in safeguarding against hot corrosion. Additionally, as enginetemperature increases, cooler areas of turbine blades, such as in theunder platform areas and the surface of internal cooling passages, whichwere previously operating at temperatures below the onset of hotcorrosion, are now becoming exposed to hotter temperature regimes atwhich Type II hot corrosion can occur. The complicated geometry in theseareas can create additional challenges for conventional line-of-sightcoating processes such as thermal spray and physical vapor deposition.Rapidly deteriorating air quality in many parts of the world,particularly throughout several countries in Asia, further compounds theproblems. Still further, hot corrosion attack often interacts with otherdegradation modes (i.e., fatigue) during service to accelerate failureof the engine components.

Environmental coatings such as nickel aluminide, platinum aluminide, orMCrAlY overlay coatings are often applied onto the airfoil of gasturbines to enhance oxidation resistance. However, such coatings do notadequately protect engine components against Type II hot corrosionattack.

One method utilized to mitigate hot corrosion attack is theincorporation of chromium onto the surface of a component by a processknown as “chromizing”. Two common industrial methods for producingchromizing coatings are pack cementation and vapor phase process.

Pack cementation requires a powder mixture including (a) a metallicsource (i.e., donor) of chromium, (b) a vaporizable halide activator,and (c) an inert filler material such as aluminum oxide. Parts to becoated are entirely encased in the pack materials and then enclosed in asealed chamber or retort. The retort is then heated in a protectiveatmosphere to a temperature between 1400-2100° F. for 2-10 hours toallow chromium to diffuse into the surface. Although the pack chromizingprocess has been used since the 1950's, there are several majorlimitations. First, the pack process generates a large amount ofhazardous waste and requires considerable more raw materials than otherprocesses. Second, the pack process is difficult to fully coat selectiveregions of the parts with complicated geometries, such as the surface ofinternal cooling passages.

The vapor phase process generally involves placing the parts to becoated into a retort in an out-of-contact relationship with a chromiumsource and halide activator. The vapor phase process can coat both theexternal and the internal surfaces of a part, such as a turbine bladehaving a complicated geometry. However, the chromium content within theresultant coating is generally too low to provide sufficient protectionagainst Type II hot corrosion attack. Furthermore, it is difficult tomask the area where no “chromizing coating” is required. Consequently,the vapor phase process has a tendency to produce a chromizing coatingalong all surfaces of the part.

Another type of chromizing process is the slurry process described inU.S. Pat. Nos. 4,904,501 and 8,262,812. In the slurry process, a thinlayer of aqueous slurry comprising chromium powder and halide activatoris directly applied to the substrate surface. The slurry processrequires much less raw materials than the pack method, and eliminatesthe exposure to dust particulates characteristic of the pack method. Oneof the major limitations of existing slurry processes is that thecoating microstructure comprises greater than or equal to 40% by volumealpha chromium (“α-chromium”), which can cause the coating to have poorfatigue crack resistance.

All of the conventional chromizing processes suffer from majordrawbacks. First, substantial amounts of oxide and nitride inclusionsare formed in the chromizing coating. The inclusions tend to reduce theerosion, fatigue and corrosion resistance of the coating. A seconddrawback is the formation of a thick and continuous alpha-chromiumlayer. Although the α-chromium layer offers excellent resistance to typeII hot corrosion attack, the α-chromium is brittle and susceptible tothermal fatigue cracking during service. The cracking can propagate intothe substrates and lead to the premature failure of the coated system.

In view of the drawbacks of existing chromizing processes there is aneed for a new generation chromizing process that can produce a chromiumenrich layer with significant reduced level of nitrides, oxides andα-chromium phase, thereby overcoming the current limitations of existingpack, vapor phase and slurry chromizing processes. Furthermore, there isa need for a simple method that can produce a chromizing coating on theselective regions and minimizes masking requirements for areas where “nocoatings” are required. There is a need for a method that utilizesconsiderable fewer raw materials and minimizes exposure of hazardousmaterials in the workplace. Other advantages and applications of thepresent invention will become apparent to one of ordinary skill in theart.

SUMMARY OF THE INVENTION

The invention may include any of the following aspects in variouscombinations and may also include any other aspect of the presentinvention described below in the written description.

In a first aspect, a slurry composition is provided, comprising: achromium source comprising elemental chromium powder, alloyed chromiumpowder, chromium-containing compounds or a mixture thereof; anon-nitrogen halide activator characterized by the absence of ammoniumhalide; a buffer material selected from the group consisting of nickel,cobalt, silicon, aluminum, silicon, titanium, zirconium, hafnium,yttrium, manganese and any combination thereof; and a binder solution,said binder solution comprising a binder material dissolved in asolvent, said solvent compatible with each of the non-nitrogen halideactivator and the binder material.

In a second aspect, a chromium diffusion coating is provided. Thecoating comprises an outer α-Cr layer comprising a thickness from about0% to about 10% of a total coating thickness; an inner nickel-chromiumlayer comprising between about 15% to about 50% chromium by weight;wherein said coating is characterized by a substantial reduction ofoxide and nitride inclusions in comparison to chromium diffusioncoatings derived from conventional slurry chromizing processes.

In a third aspect, a chromium diffusion coating is provided that isprepared by the process comprising the steps of providing a substrate;providing slurry constituents comprising: a chromium source comprisingelemental chromium powder, alloyed chromium powder, chromium-containingcompounds or a mixture thereof; a non-nitrogen halide activatorcharacterized by the absence of ammonium halide; a buffer materialselected from the group consisting of nickel, cobalt, silicon, aluminum,silicon, titanium, zirconium, hafnium, yttrium, manganese and anycombination thereof; and a binder solution, said binder solutioncomprising a binder material dissolved in a solvent; mixing saidconstituents to from a slurry composition; applying said slurrycomposition onto a metallic substrate; heating said slurry from about1600 F to about 2100 F for a duration ranging up to about 24 hours; andforming said chromium diffusion coating within said substrate.

In a fourth aspect, an article coated by the slurry composition of claim1 is provided.

BRIEF DESCRIPTION OF THE DRAWINGS

The objectives and advantages of the invention will be better understoodfrom the following detailed description of the preferred embodimentsthereof in connection with the accompanying figures wherein like numbersdenote same features throughout and wherein:

FIG. 1 shows across-sectional microstructure of a chromium diffusionlayer using a slurry composition (slurry A) which comprises an ammoniumchloride activator, whereby the resultant coating contains a significantamount of detrimental nitride inclusions, and brittle α-chromium phase;

FIG. 2 shows a cross-sectional microstructure of a chromium diffusionlayer using a slurry composition (slurry B) in accordance with thepresent invention which comprises an aluminum fluoride activator,whereby the resultant coating exhibited the reduced level of detrimentalnitride inclusions and brittle α-chromium phase in the coating;

FIG. 3 shows a cross-sectional microstructure of a chromium diffusionlayer using a slurry composition (slurry C) which comprises an ammoniumchloride activator, nickel powder, and aluminum powder, whereby theaddition of nickel and aluminum powder into slurry A only slightlyreduced detrimental nitride and oxide inclusions, and brittle α-chromiumphase in the coating.

FIG. 4 shows a cross-sectional microstructure of a chromium diffusionlayer using a slurry composition (slurry D) in accordance with theinvention which comprises an aluminum fluoride activator, nickel powder,and aluminum powder, whereby the addition of nickel and aluminum powderinto slurry B significantly reduced detrimental nitride and oxideinclusions, and brittle α-chromium phase in the coating; and

FIG. 5 shows a cross-sectional microstructure of a chromium diffusionlayer using a slurry composition (slurry E) in accordance with thepresent invention which comprises an aluminum fluoride activator andnickel powder, whereby the addition of nickel powder into slurry Bsignificantly reduced detrimental nitride and oxide inclusions, andbrittle α-chromium phase in the coating.

DETAILED DESCRIPTION OF THE INVENTION

The objectives and advantages of the invention will be better understoodfrom the following detailed description of the preferred embodimentsthereof in connection. The present disclosure relates to novel slurryformulations which produce improved chromium diffusion coatings. Thedisclosure is set out herein in various embodiments and with referenceto various aspects and features of the invention.

The relationship and functioning of the various elements of thisinvention are better understood by the following detailed description.The detailed description contemplates the features, aspects andembodiments in various permutations and combinations, as being withinthe scope of the disclosure. The disclosure may therefore be specifiedas comprising, consisting or consisting essentially of, any of suchcombinations and permutations of these specific features, aspects, andembodiments, or a selected one or ones thereof.

Generally speaking, the slurry chromizing process is considered to be achemical vapor deposition process. Upon heating to elevated temperature,the chromium source and the halide activator in the slurry mixture reactto form volatile chromium halide vapor. Transport of the chromium halidevapor from the slurry to the surface of the alloy to be coated takesplace primarily by the gaseous diffusion under the influence of chemicalpotential gradient between the slurry and the alloy surface. Uponreaching the alloy surface, these chromium halide vapors react at thesurface and deposit chromium, which diffuses into the alloy to form thecoating. As will be explained, the nature of constituents in the slurrymixture defines the thermodynamic condition of the chromizing processand dictates the final coating composition and microstructure.

A novel chromizing composition has been discovered with significantlyimproved erosion, fatigue and corrosion resistance characteristics as aresult of suppressing, minimizing or substantially eliminating oxide andnitride inclusions along with the α-chromium phase. The resultantchromium diffusion coatings of the present invention have the ability tobe locally applied to selected regions of metallic substrates, incomparison to conventional chromizing processes, and further in a mannerthat produces less material waste. Unless indicated otherwise, it shouldbe understood that all compositions are expressed as weight percentages(wt %).

The chromizing compositions of the present invention represent asubstantial improvement over conventional chromium diffusion coatingsproduced from pack, vapor or slurry processes. The improved formulationis based, at least in part, upon the selected combination of specifichalide activators and buffer materials within the slurry formulation.One embodiment of the present invention is directed to modified slurrycompositions which produce a chromium diffusion coating containingsubstantial reduced level of nitrides, oxides and alpha-chromium phase.The slurry composition comprises a chromium source, a specific class ofhalide activator, a specific buffer material, a binder material and asolvent. The slurry composition of the present invention comprises achromium source in a range from about 10% to about 90% of the slurryweight; a halide activator in a range from about 0.5% to about 50% ofthe chromium source weight, a buffer material ranging from about 0.5% toabout 100% of the chromium source; a binder solution in a range fromabout 5% to about 50% of the slurry weight in which the binder solutionincludes a binder and a solvent. An optional inert filler material maybe provided that ranges from about 0% to about 50% of the slurry weight.In a preferred embodiment, the chromium source is in a range from about30% to about 70%; the halide activator is in a range from about 2% toabout 30% of the chromium source, the buffer material is in a range fromabout 3% to about 50% of the chromium source; the binder solution in arange from about 15% to about 40% of the slurry weight; and the optionalinert filler material is in a range from about 5% to about 30% of theslurry weight.

Various chromium sources may be utilized, including elemental chromiumpowder or alloyed chromium powder or a mixture thereof. The chromiumpowder may be alloyed with other metals such as Fe—Cr, Ni—Cr, Co—Cr andCr—Si alloy powders. The chromium source may also be selected from achromium-containing compound such as Cr₃C₂. Any particle size iscontemplated by the present invention. In a preferred embodiment, thechromium source powders employed in the slurry composition have aparticle size of −200 mesh (i.e., 74 microns) or finer.

In accordance with the present invention, the activator has the abilityto readily react with the chromium source and produce chromium halidevapors and produce Cr-containing diffusion coatings without producingelevated levels of contaminant inclusions typically encountered withconventional chromizing processes. The slurry composition of thisinvention comprises a specific class of halide activators. Specifically,the present invention utilizes activators such as, by way of example,but not limited to, aluminum fluoride, chromium fluoride, aluminumchloride, chromium chloride and any combination thereof. The activatorsspecifically exclude metal halides which contain ammonium halides, asthese categories of activators adversely affect corrosion properties andmicrostructure of the coating. While the exact mechanism is not known,the prescribed halide activators appear to have a tendency to interactwith the chromium source yet still maintain chromium activity at a levelthat does not generate enriched α-chromium phase.

As previously mentioned, the halide activators of the present inventionare present in the slurry composition in an amount of about 0.5% toabout 50%, and more preferably from about 2% to about 30% of the weightof the chromium source. It has been discovered that incorporating theactivator in an amount below 0.5% of chromium source can produce a thinchromizing coating with low chromium content, thereby impartinginadequate corrosion resistance. The presence of the activators inexcess of 50% of the chromium source appears to confer no additionalbenefit and may in some instances attack the coating.

The halide activator in the inventive slurry generates volatile chromiumhalide vapors by reacting with the chromium source powder at elevatedtemperatures. The chromium halide vapors can then transport to thesurface of a metallic substrate and produce the desired coatingcomposition and microstructure by solid state diffusion. As will beshown in the Examples, the specific type of halide salt selected as theactivator in the slurry mixture can impact the final coatingmicrostructure and coating composition. In particular, it has beendiscovered that metal halides which contain ammonium halides create poorcoating compositions having nitride inclusions. Ammonium halides, suchas ammonium chloride, are commonly used in the conventional chromizingprocess due to their activation effectiveness (i.e., ability to readilyreact with the chromium source and produce chromium halide vapors).However, without being bound by particular theory, the use of anammonium halide activator may promote the formation of substantialamounts of nitride inclusions within the coating, which cansignificantly degrade the corrosion, erosion and fatigue resistance ofthe coating. Upon heating, ammonium halides can rapidly decompose intonitrogen, hydrogen and halogen gases. While halogen gas reacts withchromium source to form volatile chromium halide vapor and form acoating on a metallic substrate, nitrogen from the decomposition ofammonium halides can react with active elements, such as aluminum andtitanium, in the metallic substrate and form internal nitride inclusionswithin the coating.

Besides nitride formation in the coating, the rapid decomposition ofammonium halides also generates undesirable high pressure in the coatingretort which can pose a safety risk during the coating operation. Theprocess variables such as gas flow through the container or amount ofactivator can be adjusted to reduce pressure. However, while suchadjustments reduce the amount of nitride phases in the coating, theresultant coating thickness and/or composition is compromised.

Accordingly, the present invention utilizes a non-nitrogen containinghalide activator so as to suppress, substantially reduce or eliminatethe amount of internal nitride inclusions in the coating. A non-nitrogencontaining halide activator also results in significantly lower levelsof deleterious α-chromium phase along the outer region of the coating.

In another embodiment of the present invention, the halide activatorexcludes nitrogen, alkali metal halides, such as sodium chloride, andalkaline earth metal halides such as magnesium chloride. Although alkalimetal halides and alkaline earth metal halides exhibit higher stabilitythan ammonium halides, the present invention recognizes that alkaline oralkaline earth metal elements may in some applications have a tendencyto be incorporated into the resultant chromizing coating during thecoating process. Incorporation of the alkali metal halides or alkalineearth metal halides in some instances may adversely affect the corrosionproperties of the coating.

In addition to selection of the proper activator being present at theprescribed optimal range in the slurry, the slurry composition of thepresent invention is further defined by the proper selection of one ormore additional buffer powders (i.e., buffer material as listed in Table1). The buffer material may include nickel, cobalt, silicon, aluminum,silicon, titanium, zirconium, hafnium, yttrium, manganese and anycombination thereof in a range from about 0.5% to about 100%, and morepreferably from about 5% to about 80% of the weight of the chromiumsource. The buffer material has a high affinity for oxygen and nitrogen,and can therefore effectively getter residual nitrogen and oxygen in theslurry and retort atmosphere. Furthermore, the buffer lowers thechemical activity of chromium in the slurry to a level which suppressesor reduces the level of brittle α-chromium phase in the outer layer ofthe chromizing coating, but which maintains sufficient chromium chemicalactivity to form the necessary chromium within the inner layer. In thismanner, the synergistic combination of the buffer material with suitablehalide activator in accordance with the principles of the presentinvention reduces the level of nitride and oxide inclusions while alsolowering α-chromium phase in the coating to levels not attainable bycoatings produced from conventional pack, vapor or slurry chromizingprocesses.

Careful selection of the buffer material in combination with the halideactivator in accordance with principles of the present invention isrequired to generate improved chromium diffusion coatings. As will beshown by the Examples, the superior coating characteristics of thepresent invention are not solely based on the buffer material, but alsoselection of a suitable halide activator that is compatible with thebuffer material. Further, the halide activator is contained in optimalamounts within the slurry formulation. Under such conditions, the halideactivator synergistically interacts with the buffer material to allowthe levels of nitride, oxide and α-chromium phase in the coating to besuppressed, minimized or substantially eliminated. In this regard, acomparison of Example 1 and Comparative Example 3, each of which will bediscussed below in greater detail, shows that although the slurryformulation of Comparative Example 3 utilized a nickel and aluminummetallic powder mixture, the proper type of halide (i.e., exclusion ofnitrogen containing halide activators) was not incorporated. As aresult, the coating of Comparative Example 3 was inferior to Example 1,which utilized both the nickel and aluminum powder mixture along with analuminum fluoride activator. The interaction of these and otherconstituents in the slurry formulation of Example 1 facilitatedgeneration of significantly lower levels of nitride, oxide andα-chromium phase in the resultant coating.

The slurry composition of the present invention further comprises abinder solution, which contains a binder material dissolved in asolvent. The binder solution functions to hold the slurry constituentstogether without detrimentally interfering with the slurry constituentsor the coated substrate. The binder must be capable of burning offcleanly and completely without interfering with the chromizingreactions. A preferred binder is hydroxypropylcellulose, which iscommercially available under the trade name Klucel™, from AshlandIncorporation. Other binders may also be suitable for the presentinvention, including by way of example. a B-200 binder commercially madeand sold by APV Engineered Coatings (Akron, Ohio). The selected binderexhibits compatibility with the halide in the slurry composition orformulation. In particular, the halide activator does not react with thebinder material and solvent, nor affect the physical and chemicalproperties of the binder solutions. For example, if a water-based bindersolution was used, the particular halide activator that is selectedpreferably exhibits negligible solubility in water. Otherwise, therelatively high concentrations of dissolved halide activator in thewater-based binder solution may have a tendency to cause the binder togradually precipitate out of the water-based binder solution, therebyleading to a short shelf-life of the slurry.

The solvent employed in the slurry coating compositions of the presentinvention is chosen such that its volatility, flammability, toxicity andcompatibility with both halide activator and binder are taken intoconsideration. In a preferred embodiment, the solvent includes deionizedwater. The amount of binder solution accounts for about 5% to about 50%,and more preferably from about 15% to about 40% of the weight of theslurry.

The slurry composition optionally comprises a filler that can range fromabout 0% to about 50%. The filler material is chemically inert. Theinert filler material does not participate in the chemical reactions inthe slurry. Instead, the filler material is designed to impart adilution effect to the slurry mixture. The inert filler material canalso adjust the viscosity of the slurry mixture. In a preferredembodiment, alumina powder is utilized as the inert filler material.Other types of filler materials can be utilized, such as silica andkaolin.

The slurries of the present invention have demonstrated long shelf-livesthat range at least 3 months, and more preferably at least 6 months withregards to the binder material remaining in the solvent and the solidcontents remaining unreactive and stable in the binder solution.

The slurry compositions of the present invention can be applied to ametallic substrate by conventional methods such as brushing, spraying,dipping and injecting. The method of application depends, at least inpart, on the viscosity of the slurry composition, as well as thegeometry of the substrate surface. The slurry can be applied either toall surfaces of the substrate, or only to the selective regions of asubstrate without specific tooling requirements. Advantageously, theability to locally apply the slurry to only desired regions of themetallic substrate eliminates the need to utilize masking techniques.

The slurry composition is applied onto the metallic substrate and driedeither with warm air in a convection oven, or under infrared lamp or thelike. The slurry-coated substrate is then heated to 1600° F.-2100° F.for a duration ranging up to about 24 hours, and more preferably fromabout 2 hours to about 12 hours to allow the formation of chromiumdiffusion coating. During the processing, adequate flow of argon,hydrogen or the mixture is maintained to purge substantially all of thebinder outgassing from the retort.

After processing, slurry residues can be removed by various methods,including wire blush, oxide grit burnishing, glass bead, high-pressurewater jet or other conventional methods. Slurry residues typicallycomprise unreacted slurry compositional materials. The removal of anyslurry residue is conducted in such a way as to prevent damage to theunderlying chromizing surface layer.

Preferably, the slurry coating compositions of the invention areformulated for application onto nickel-based, cobalt-based or iron-basedalloys. A nickel based alloy, for example, is an alloy having a matrixphase having nickel as the proportionally largest elemental constituent(by weight). Other elements such as aluminum may be added to the nickelbased alloy to impart improvements in physical or chemical properties.

The chromizing coating consists of two layers: an outer α-Cr layercontaining above 70% Cr, by weight, and an inner Ni(Cr) layer defined aschromium in a solid solution of nickel. In accordance with theprinciples of the present invention, the combination of a specificactivator and a specific buffer material at certain levels interactswith each other to facilitate formation of a chromizing coating whichcontains a significantly reduced level of nitride, oxide inclusions andα-chromium phase. The inner Ni(Cr) layer contains a nickel-chromiumphase comprising about 15% to about 50% chromium by weight, morepreferably about 25% to about 40%. The chromium content in the Ni(Cr) issufficient to impart the desired corrosion resistance for variousend-use applications, including aerospace applications. The thickness ofthe outer α-chromium layer coating is reduced over conventional chromiumdiffusion coatings to only account for about 0% to about 40%, and morepreferably from about 0% to about 10% of the total coating thickness,thereby allowing the coating to maintain adequate fatigue resistancewhile eliminating brittleness typically encountered with large amountsof α-chromium layer formed in the outer layer.

The examples below demonstrate the unexpected improvements in utilizinga modified slurry formulation to form chromium diffusions coatings ofthe present invention in comparison to conventional coatings.

Comparative Example 1

A slurry composition, designated “Slurry A”, was prepared by aconventional formulation typically used in conventional pack, vapor, orslurry chromizing processes. Slurry A comprised elemental chromiumpowders and an ammonium chloride activator. Slurry A was prepared bymixing the following: 100 g chromium powder, −325 mesh; 5 g ammoniumchloride (halide activator); 4 g Klucel™ hydroxypropylcellulose(binder); 51 g deionized water (solvent); and 40 g alumina powder (inertfiller material).

The slurry A was applied onto the surface of a Rene N5 specimen bydipping. Rene N5 is a single crystal nickel-based superalloy having anominal composition of, by weight, about 7.5% Co, 7.0% Cr, 6.5% Ta, 6.2%Al, 5.0% W, 3.0% Re, 1.5% Mo, 0015% Hf, 0.05% C, 0.004% B, 0.01% Y, thebalance nickel.

The slurry coating was allowed to dry in an oven at 80° C. for 30minutes followed by curing at 135° C. for 30 minutes. The coatedspecimen was then diffusion heat-treated in a flowing argon atmosphereat 2010° F. for 4 hours. After cooling, the slurry residues were removedfrom the surface of the specimen by grit blasting with 220 mesh alumina.

The coated specimen was cross-sectioned for metallurgical analysis. FIG.1 shows the resultant coating microstructure. The results are summarizedin Table 1.

Two microstructure characteristics were observed in FIG. 1, which isvery similar to chromizing coatings formed by conventional pack, vapor,or slurry chromizing process. First, the coating contained a continuousouter α-chromium layer. The thickness of the α-chromium layer accountedfor 40% of total coating thickness. Such a thickness along the outerregion of the region generated unacceptable brittleness that isdetrimental to the mechanical performance of the coated specimen.Second, the coating was observed to contain significant amounts ofinternal nitride and oxide inclusions, which can degrade the corrosionand erosion performance of the coating. Aluminum oxide inclusions wereprimarily interspersed in the outer α-chromium layer of the coatingwhile aluminum nitride inclusions were located in the inner layer ofnickel-chromium solid solution. White arrows in FIG. 1 indicated thealuminum nitride inclusions in the form on angular inclusions in theinner layer of the coating. The nitride phase is marked with whitearrows in FIG. 1.

The volume fraction of nitride and oxide inclusions was measured by anautomatic image analyzer in a manner as specified by ASTM E1245. Theinclusions were to be 14.5%.

Comparative Example 2

A second slurry composition, designated “slurry B”, was prepared inaccordance with the present invention by replacing the ammonium chlorideactivator in slurry A with an aluminum fluoride activator. The slurry Bcontained: 100 g chromium powder, −325 mesh; 20 g aluminum fluoride(halide activator); 4 g Klucel™ hydroxypropylcellulose (binder); 51 gdeionized water (solvent); and 25 g alumina powder (inert filler).

Slurry B was applied to a Rene N5 specimen and diffusion-treated in anargon atmosphere at 2010° F. for 4 hours, as set forth in ComparativeExample 1. The coated specimen was cross-sectioned for metallurgicalanalysis. The results are summarized in Table 1.

FIG. 2 shows the resultant coating microstructure that was produced. Thedeleterious α-chromium phase was reduced in comparison to ComparativeExample 1. Specifically, the thickness of the outer α-chromium layerusing slurry B only accounted for 14% of the total coating thickness,compared to 40% using slurry A in Comparative Example 1.

It was observed that the amount of internal nitride inclusions in thecoating was significantly reduced by replacing the ammonium chloride inslurry A with aluminum fluoride in slurry B, thereby eliminating anitrogen precursor source for nitride formation in the coating. Thevolume of nitride and oxide inclusions in the coating was reduced from14.5% using slurry A (Comparative Example 1) to 11.6% using slurry B.Nonetheless, the amount of inclusions was determined to be unacceptablyhigh so as to result in poor erosion, corrosion and fatigue resistanceof the coating.

Comparative Example 3

Tests were performed to assess the microstructure and composition of acoating prepared from a slurry formation typically utilized when formingcoatings from standard pack processes. In this regard, ammonium chlorideand a buffer material containing a mixture of nickel and aluminumpowders were incorporated into the slurry composition. The slurrycomposition, designated “Slurry C”, was prepared by mixing thefollowing: 70 g chromium powder, −325 mesh; 5 g ammonium chloride(halide activator); 4 g Klucel™ hydroxypropylcellulose (binder); 51 gdeionized water (solvent); 25 g nickel powder and 5 g aluminum powder(metallic buffer powder); and 40 g alumina powder (inert fillermaterial).

Slurry C was applied to a Rene N5 specimen and diffusion-treated in anargon atmosphere at 2010° F. for 4 hours as set forth in ComparativeExample 1. The coated specimen was cross-sectioned for metallurgicalanalysis. The results are summarized in Table 1.

FIG. 3 shows the resultant coating microstructure. The addition ofnickel and aluminum powder reduced the amount of nitride and oxideinclusions in the coating to 13.2% using slurry C in comparison to thecoating produced from Slurry A of Comparative Example 1, which exhibiteda volume fraction of 14.5% of inclusions. The addition of nickel andaluminum powder only slightly reduced the fraction of deleteriousα-chromium phase, from 40% by thickness using slurry A to 30% bythickness using slurry C. The results indicated that the ammoniumchloride negatively impacted the coating and offset any benefitsprovided by the buffer material. It was determined from the test that apack formulation could not be successfully utilized in a slurrychromizing process to produce clean coatings with favorablemicrostructure (i.e., absence of nitride and oxide inclusions andreduced alpha-chromium).

Example 1

Tests were performed to assess the microstructure and composition of acoating prepared from a slurry formation that replaced the ammoniumchloride activator in Slurry C with an aluminum fluoride activator. Inthis regard, “Slurry D”, was prepared by mixing the following: 70 gchromium powder, −325 mesh; 20 g aluminum fluoride (activator); 4 gKlucel™ hydroxypropylcellulose (binder); 51 g deionized water (solvent);25 g nickel powder and 5 g aluminum powder (buffer material); and 25 galumina powder (inert filler material).

Slurry D was applied to a Rene N5 specimen and diffusion-treated inargon atmosphere for 4 hours as set forth in Comparative Example 1. Thecoated specimen was cross-sectioned for metallurgical analysis. Resultsare summarized in Table 1.

FIG. 4 shows the resultant coating microstructure. It was observed thatthe combination of aluminum fluoride activator, nickel and aluminumpowder led to a significant reduction of nitride and oxide inclusions,as well as the α-chromium phase in the coating. The resultant coatingcontained insignificant amounts, 2.6% by volume, of nitride and oxideinclusions, compared to 13.2% using slurry C (Comparative Example 3),and 11.6% using slurry B (Comparative Example 2). Furthermore, thethickness of the outer α-chromium layer accounted for 4% of totalcoating thickness, compared to 30% using slurry C or 14% using slurry B.The results indicated that a non-nitrogen halide activator favorablyinteracted with the buffer material during formation of the diffusioncoating, and, as a result, both the correct halide activator and buffermaterial was required to produce improved coatings.

Example 2

Further tests were performed to evaluate a coating composition andmicrostructure prepared from a slurry containing a non-nitrogen halideactivator and metallic buffer powder containing nickel. In this regard,a slurry composition, designated “slurry E”, was prepared in accordancewith the present invention by removing the aluminum powder from slurryD. Slurry E was prepared by mixing the following: 75 g chromium powder,−325 mesh; 20 g aluminum fluoride (halide activator); 4 g Klucel™hydroxypropylcellulose (binder); 51 g deionized water (solvent); 25 gnickel powder (buffer material); and 25 g alumina powder (inert fillermaterial).

The slurry E was applied to a Rene N5 specimen and diffusion-treated inargon atmosphere for 4 h as set forth in Comparative Example 1. Thecoated specimen was cross-sectioned for metallurgical analysis. Resultsare summarized in Table 1.

FIG. 5 shows the resultant coating microstructure. The results werecomparable to that of Example 1. The combination of aluminum fluorideactivator and nickel powder led to the significant reduction of nitrideand oxide inclusions, and α-chromium phase in the coating. The resultantcoating contained insignificant amounts, 2.5% by volume, of nitride andoxide inclusions, compared to 13.2% using slurry C (Comparative Example3), and 11.6% using slurry B (Comparative Example 2). Additionally, thethickness of the outer α-chromium layer accounted for less than about 2%of total coating thickness, compared to 30% using slurry C or 14% usingslurry B.

TABLE I Slurry Composition and the Resultant Coating MicrostructureCoating characterization Volume Average Cr Slurry Formula fraction ofThickness content in Buffer inclusions, percent of α-Cr ANi(Cr) layer,Slurry Activator material % layer, % wt. % A NH₄Cl — 14.5 40% 20-25% BAlF₃ — 11.6 14% 25-40% C NH₄Cl Ni, Al 13.2 30% 20-25% D AlF₃ Ni, Al 2.6<4% 25-40% E AlF₃ Ni 2.5 <2% 25-40%

As has been shown, the present invention offers a unique slurryformulation that produces chromium diffusion coatings that areadvantageous over chromium diffusion coatings produced from conventionalchromizing slurry, pack and vapor phase processes. In particular, theExamples demonstrate that the present invention produces superiorchromium coating composition and microstructure (i.e., reducedinclusions and reduced α-chromium) in comparison to those produced fromconventional slurry chromizing processes. As a result, the coatings ofthe present invention have improved properties, including higherresistance to corrosion, erosion and fatigue.

Further, the slurries of the present invention are advantageous in thatthey can be selectively applied with control and accuracy onto localizedregions of the substrate by simple application methods, includingbrushing, spraying, dipping or injecting. On the contrary, conventionalpack and vapor phase processes cannot locally generate chromium coatingsalong selected regions of a substrate. As a result, these conventionalcoatings require difficult masking techniques which typically are noteffective in concealing those regions along the metallic substrate notdesired to be coated. To overcome masking challenges, chromizing vaporand pack processes utilize a post-coating machining step to removeexcess coating from undesired surfaces of the metallic substrate.

The ability for the present invention to locally apply slurryformulations to form coatings has the added benefit of significantlylower material waste. As such, the present invention can conserveoverall slurry material and reduce waste disposal, thereby creatinghigher utilization of the slurry constituents. No masking is required,thereby reducing the raw materials required for coating and minimizingexposure of hazardous materials in the workplace. On the contrary, packprocesses typically require significantly higher amounts of materialthat results in more waste material. Similar deficiencies exist forvapor phase processes.

Still further, unlike pack and vapor phase processes, the modifiedslurry formulations of the present invention can be used to form theimproved chromium coatings onto various parts having complex geometriesand intricate internals. Pack and vapor processes have limitedversatility, as they can only be applied to parts having a certain sizeand simplified geometry.

The principles of the present invention may be utilized to coat anysuitable substrate requiring controlled application of chromizingcoatings. In this regard, the methods of the present invention canprotect a variety of different substrates that are utilized in otherapplications. For example, the chromizing coatings as used herein may belocally applied in accordance with the principles of the presentinvention onto stainless steel substrates which do not containsufficient chromium for oxidation resistance. The chromizing coatings insuch applications form a protective oxide scale along the stainlesssteel substrate.

While it has been shown and described what is considered to be certainembodiments of the invention, it will, of course, be understood thatvarious modifications and changes in form or detail can readily be madewithout departing from the spirit and scope of the invention. It is,therefore, intended that this invention not be limited to the exact formand detail herein shown and described, nor to anything less than thewhole of the invention herein disclosed and hereinafter claimed.

1. A chromium diffusion coating, comprising: an outer α-Cr layercomprising a thickness from about 0% to about 10% of a total coatingthickness; an inner nickel-chromium layer comprising between about 15%to about 50% chromium by weight; wherein said coating is characterizedby a substantial reduction of oxide and nitride inclusions in comparisonto chromium diffusion coatings derived from conventional slurrychromizing processes.
 2. The chromium diffusion coating of claim 8,wherein said inclusions comprise less than about 3% volume fraction. 3.The chromium diffusion coating of claim 8, wherein said outer α-Cr layercomprises a thickness of less than about 4% of a total coatingthickness.
 4. The chromium diffusion coating of claim 8, said coatingapplied onto a substrate along selected regions.
 5. The chromiumdiffusion coating of claim 8, wherein said outer α-Cr layer comprises athickness of less than about 2% of a total coating thickness.